Automobiles are known to include fuel tank module assemblies as part of the fuel system. The module assembly may include a fuel pump and a fuel level sensor, along with a pressure sensor. For the pressure sensor, it is known to provide a hole in the fuel tank module and a pressure sensor assembly installed in the hole. The pressure sensor assembly includes an outer body or housing, which may be of plastic, and defines a bore extending therethrough. The housing further defines a pocket in flow communication with the bore. A pressure sensor chip is installed in the pocket to sense pressure changes via the bore. Circuitry in the chip can be used to sense flexing of a diaphragm-like structure on a surface of the chip in response to pressure changes in the bore. Oven-cured epoxy has been used to secure the chip in the pocket.
It can be problematic to adhere a sensor chip directly to a plastic sensor assembly housing. Plastic is susceptible to thermal expansion during operation and/or as a result of changes in the ambient environment, Expansion and contraction of the plastic housing can interfere with the bond to the sensor chip. Accordingly, it has been known to provide a glass barrier having an aperture therethrough in fluid flow communication with the bore in the housing. The glass barrier is secured in the bottom of the pocket as an intermediate component between the plastic housing and the sensor chip. Barriers of this type have been referred to as a “bead”. The glass barrier is secured in the bottom of the pocket formed in the housing, and the pressure sensing chip is attached over the aperture in the glass barrier to be in fluid flow communication with the bore in the sensor assembly housing.
The use of a glass barrier as described above can also have disadvantages under some conditions. Different thermal expansion rates between the glass and the plastic housing can cause operational difficulties under some conditions. Metal barriers have been found as suitable replacements for glass barriers, to overcome some of the operational difficulties from thermal expansion and contraction of glass beads. A metal barrier, also referred to as a “bead”, is secured in the bottom of the housing pocket in the same way as glass beads have been, and includes an aperture in flow communication with the bore in the housing.
It has been known for leakage to occur around metal barriers or beads in automobile fuel systems, interfering with proper operation of the pressure sensor system. In a sensing device as described, leakage of air, fuel vapor or the like can cause the sensor device to function improperly due to the loss of pressure or the loss of vacuum due to venting passed the barrier. Leakage has been found to occur apart from any abnormal physical changes due to expansion or contraction of the housing or the metal bead. Since the occurrence of leakage has been somewhat random, it has been necessary to pressure test all assemblies before installation.